Like most folks I use a central directory that is shared by all my domains.
I think you will need to set this up with your host. It will be located somewhere on your server/webspace, and will be able to be used by any of your domains, as if it were a subfolder.
Say the directory is called /global/ - well, any of your domains can use it as if it were
www.domain.com/global/
Very useful indeed
